This invention relates generally to electromagnetic spring-wound clutches, such as for use in controlling the transmission of power from an automobile engine to a refrigerant compressor in an automobile air conditioning system, and more particularly, to an improved connecting structure between an armature plate and a hub for transferring the rotation of a pulley to the drive shaft of the compressor.
The general structure of a conventional electromagnetic spring-wound clutch for use between an automobile engine and an air conditioning compressor is disclosed in U.S. Pat. No. 3,735,847 and shown in FIG. 1. This electromagnetic spring-wound clutch includes pulley 1 rotatably supported on a tubular extension of a compressor housing through bearing 2, hub 3 fixed on the outer end of a drive shaft and armature plate 4 placed between pulley 1 and hub 3 facing the axial end surface of pulley 1 at a predetermined gap. Pulley 1 has axial flange 1a projecting outwardly in an axial direction from an axial end surface thereof. Coil-wound spring 5 is disposed around axial flange 1a of pulley 1 at a predetermined gap, and has its respective ends connected to armature plate 4 and hub 3. Electromagnetic coil 6 is coaxially disposed inside pulley 1 at a small gap for attracting armature plate 4 to pulley 1. When electromagnetic coil 6 is energized, the rotating motion of pulley 1 is transmitted to the drive shaft of the compressor by winding up or tightening coil-wound spring 5 on flange 1a, and when electromagnetic coil 6 is not energized, coil-wound spring 5 is unwound so that, while pulley 1 is rotated by the automobile engine, the compressor is not driven.
In the above construction, armature plate 4 is joined by the coil-wound spring to hub 3, which is fixed on the drive shaft of the compressor. As a result, an impact force occurs at the moment when armature plate 4 contacts the axial end surface of the pulley, which causes the coil-wound spring to wind around the axial flange of the pulley and begins to suddenly transmit rotational force. However, the rigid connection of the coil-wound spring between the armature plate and the hub does not cushion the impact force. The sudden torque change is directly transmitted to the hub and the drive shaft of the compressor. The repeated occurrence of this impact force can damage the portion of the electromagnetic clutch connecting the hub and drive shaft, and also can cause a shearing stress by twisting the drive shaft, which eventually may break the drive shaft. Thus, the occurrence of this force in conventional electromagnetic clutches reduces the endurance of the clutch and compressor.